B757 flight crew reported diverting to an alternate after loss of automatic pressurization control.

Attributes
ACN	1400870
Time
Date	201611
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Pressurization Control System
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 150 Flight Crew Type 542
Person 2
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 200 Flight Crew Type 2231
Events
Anomaly	Aircraft Equipment Problem Less Severe

Narrative:

[In cruise flight] FL310 we noticed the cabin altitude climbing above normal level. After reference of checklists etc we were still unable to control the cabin altitude from climbing. We descended to FL250 and subsequently descended to FL230. This stabilized the cabin altitude just below 8000 feet. The cabin continued to descend with our lower cruise altitude. We contacted dispatch; [maintenance] and after careful consideration of factors and available resources (checklists; weather; notams; fuel; aircraft state; maintenance; performance; passengers; crew) we elected (with dispatch and [maintenance] concurrence) to divert to [a suitable airport]. We continued a gradual early descent in order to lower the cabin altitude in a comfortable manner. No sops or fars violated. No discomfort issues with passengers or crew. ATC was notified of our condition. We made a normal uneventful approach and landing.

Original NASA ASRS Text

Title: B757 flight crew reported diverting to an alternate after loss of automatic pressurization control.

Narrative: [In cruise flight] FL310 we noticed the cabin altitude climbing above normal level. After reference of checklists etc we were still unable to control the cabin altitude from climbing. We descended to FL250 and subsequently descended to FL230. This stabilized the cabin altitude just below 8000 feet. The cabin continued to descend with our lower cruise altitude. We contacted Dispatch; [Maintenance] and after careful consideration of factors and available resources (checklists; weather; NOTAMs; fuel; aircraft state; maintenance; performance; passengers; crew) we elected (with Dispatch and [Maintenance] concurrence) to divert to [a suitable airport]. We continued a gradual early descent in order to lower the cabin altitude in a comfortable manner. No SOPs or FARs violated. No discomfort issues with passengers or crew. ATC was notified of our condition. We made a normal uneventful approach and landing.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.